As the loading of a DC motor increases, the current draw and internal heat dissipation of the motor also increase. Concurrently, its speed, and the amount of cooling air provided by an internal or external fan turning at motor speed, decreases. Accordingly, the temperature of the motor increases. Because temperature affects both the performance and the durability of a motor, motors are designed to operate within carefully defined loading limits. In service, however, unforeseen circumstances may cause the loading of a motor to exceed the design values, and as a result motor temperature may exceed the acceptable limit. Sometimes this results in motor failure. In extreme cases, combustion of one or more materials within the motor occurs.
An example of an application where motor overload can be a problem is that of automotive engine-cooling. Many automotive vehicles use an electric fan assembly to move air through a radiator, condenser, or other heat exchanger. Typically, a DC motor powers such an assembly. The presence of snow, ice, or mud can increase the torque required to turn the fan, or in extreme cases prevent the fan from turning at all. A failed motor bearing can likewise retard or prevent rotation. These situations can result in a failed motor. Occasionally, they can result in an under-hood fire. It is, therefore, desirable to detect an overload condition before it results in excessively high motor temperatures.
Overload detection typically involves repetitively monitoring the motor load while the motor is in use. Traditional methods of monitoring a motor load include measuring the current drawn by the motor, the motor temperature, the speed of the motor, or the back-electromotive force (“back-EMF”) generated by the motor.
Measuring back-EMF involves the intermittent de-energizing of the motor, and the measurement of the voltage of the disconnected or de-energized motor lead after sufficient time has passed for the currents in the motor to decay to zero, but before the motor has decelerated appreciably. This voltage is an indication of the back-EMF, which increases with motor speed. If the back-EMF is sufficiently low, an overload condition is indicated, and the motor can be shut down. If the back-EMF is sufficiently high, normal operation is indicated, and the circuit can be re-energized.
However, a disadvantage of current back-EMF measurement methods used to determine if an overload condition exists is the time required to make the measurement. After the motor is de-energized, some time is required for the measured voltage to approach the asymptotic value indicative of the back-EMF. This is compounded by the fact that the measurement is an inherently noisy one. The voltage of the disconnected motor lead will have a voltage ripple at the commutation frequency as well as random noise. Both of these components will vary as the condition of the brushes and commutator change over the life of the motor. To remove their effect on the voltage measurement, the signal can be filtered with a time constant equal to several commutation periods. This increases the length of time necessary to make an accurate back-EMF measurement, and the length of time that the motor is de-energized. If the de-energized periods are long enough, they can become acoustically perceptible, and can cause increased wear due to backlash between the motor shaft and the driven load. Thus, improved methods and devices of monitoring motor load are desired.